The present invention relates to a method for automatic or semiautomatic selection of a better starting gear in a vehicle.
The invention also relates to a vehicle comprising a control unit arranged to perform steps for automatic or semiautomatic selection of a better starting gear.
The invention also relates to a computer program and computer program products for carrying out said method.
Automatic Mechanical Transmissions (AMT:s) have become ever more common in heavier vehicles with the increasing development of microprocessor systems, making it possible, with a control computer and a number of control devices, such as servomotors, for example, to precisely regulate the engine speed, engagement and disengagement of an automatic disc clutch between engine and gearbox, and gearbox clutch members in relation to one another, so that smooth gear changes are always achieved at the correct engine speed.
A gearbox of the AMT-type usually comprises an input shaft, an intermediate shaft, which has at least one toothed gear meshing with a toothed gear on the input shaft, and main shaft with toothed gears, which mesh with toothed gears on the intermediate shaft. The main shaft is then further connected to an output shaft coupled to the driving wheels via a propeller shaft, for example. Each pair of toothed gears has a different gear ratio from another pair of gears in the gearbox. Different transmission ratios are obtained in that different pairs of gears transmit the torque from the engine to the driving wheels.
The development of computer technology has also had an impact on electronic control and feedback systems for a vehicle engine, and these systems have become more precise, faster and more adaptable to prevailing engine and environmental conditions. The entire combustion process can be precisely controlled according to any operating situation. The vehicle's throttle lever (an accelerator pedal, for example), which primarily controls the fuel supply to the engine, controls the vehicle's engine via electrical wiring and electronic signals. The throttle lever is therefore equipped with sensors for detecting the throttle lever position, that is to say what throttle opening is required.
In the process of vehicle take off an automatic disc clutch as described above is included in the AMT and is usually controlled by means of information on the position of the throttle lever, the rotational speed of the engine, the engine output torque, the position of the disc clutch and from a reference position of the disc clutch, which is selected on the basis of when the disc clutch just begins to transmit torque (traction position), this position being relatively easy to define. A torque typically transmitted in the reference position may be in the order of 30 Nm. The engine output torque is mostly calculated from the quantity of fuel injected into the engine. In the starting sequence and maneuvering the vehicle is typically started from stationary or a virtually stationary state, and the engine commences the sequence at idling speed. The position of the disc clutch and hence the degree of engagement, which determines the torque transmitted from the engine to the gearbox, primarily depends on where the driver positions the throttle lever. The reference position of the disc clutch is used to give the driver of the vehicle more accurate control of the vehicle take off in that the disc clutch assumes the reference position immediately a starting gear is selected and engaged. The vehicle is prepared for immediate take off. Thus in many situations the driver experiences only a small flat spot, if any, when the accelerator pedal is depressed. The driver therefore obtains a theoretically direct response and the vehicle in principle starts to move as soon as the accelerator pedal starts to be depressed.
The selection of an appropriate starting gear in an AMT is normally based on a calculation from at least parameters such as road inclination and vehicle weight. Even though this calculation covers a lot of vehicle starting situations a selected starting gear will sometimes not become right due to a changed vehicle condition, which the calculation cannot foresee. One such situation can be when a long-distance truck has been parked with the truck on a horizontal part of the road and the trailer in a down slope. From a starting gear selection point of view this situation can get even worse if the trailer and truck when parked are empty and then the trailer is loaded before a take-off. Such a calculation will probably not take into account the weight change of the trailer and that it is parked in a down slope. This will most likely make the AMT to select a too high starting gear and an unsuccessful vehicle take off is most likely to happen due to the weight increase of the trailer and in that the trailer is parked in a down slope that counteracts propulsive power. The driver will have to interrupt an unsuccessful take off attempt and hinder the truck from rolling backwards by, for example, activating the service brakes. Then a more proper starting gear will have to be selected manually since the AMT control unit will not know about the changed condition.
WO0242108 discloses and example of a starting gear selection procedure where successive downshifting is performed if not sufficient vehicle acceleration can be performed with a first engaged starting gear.
There is therefore a need to make it easier for the driver to handle such take offs as mentioned above.
The method according to an aspect of the invention describes a method for automatic or semiautomatic selection of a better starting gear in a vehicle. The method is characterized by the following steps executed in mentioned order:                selecting and engaging a first vehicle starting gear at vehicle stand still,        on driver demand controlling vehicle propulsion torque by gradually engaging a propulsion unit to driven wheels of the vehicle as to attempt to achieve a first vehicle take off,        detecting if said first vehicle starting gear is too high for a prevailing condition of the first vehicle take off,        if the first vehicle starting gear is too high then interrupting the first vehicle take off and automatically braking the vehicle,        disengaging said first vehicle starting gear,        selecting and engaging a second adapted starting gear which is a lower gear compared to said first starting gear and having a higher gear ratio,        controlling vehicle propulsion torque by gradually engaging said propulsion unit to said driven wheels as to attempt to achieve a second vehicle take off,        and if said second vehicle take off attempt results in a transmitting of torque to driven wheels of the vehicle being enough as to perform a take off then stop braking the vehicle.        
The advantage of the method according to an aspect of the invention is that a wrongly selected starting gear is automatically or semi automatically changed. A further advantage is that the starting gear change can be performed in an uphill situation. Thus, it will be possible to change the wrongly selected starting gear without risking that the vehicle is rolling off in the wrong direction.
According to an advantageous second embodiment of the method according to an aspect of the invention the step of detecting that said first vehicle take off is not possible with said first starting gear is done by calculating heat energy developed in the clutch during the first vehicle take off attempt and interrupting said first vehicle take off attempt if said heat energy exceeds a predetermined heat energy limit. This is an advantageous way of detecting if a starting gear was wrongly selected.
According to an advantageous further embodiment of the method according to an aspect of the invention the step of detecting that said first vehicle take off is not possible with said first starting gear can be done by the driver. The subsequent step of interrupting the first vehicle take of can be performed manually by the driver. This gives increased operational freedom to the driver. The step of interrupting the first vehicle take off can be performed by the driver manually selecting a lower gear by, for example, operating a gear shift lever.
According to an advantageous further embodiment of the method according to an aspect of the invention if said second take off attempt results in transmission of torque to the driven wheels enough to keep the vehicle at least in stand still, then continuing controlling clutch engagement of a clutch until the clutch is fully engaged and the vehicle takes off. This point in the engagement is a further indication of a successful starting gear selection and clutch engagement process.